Is there a reason why Amtrak would continue to use freight rail lines vs SSL/Electric district ROW? Sure they would use diesel rollingstock for the immediate future, but I cant imagine the CN track is a higher quality or is their dispatch treating them well. I also don't think that the Electric district is constrained by capacity especially given that Amtrak wouldn't be running 10 extra trains per day on that track. Getting Amtrak to use until Kensington and then switching over to SSL for all their east bound trains eliminates so many issues for them in terms of reliability and Metra could very easily request Amtrak make a number of key track improvements prior to getting access to that line.
That line to Kensington is so straight, I don't think you should be going below 79mph for any of it unless you are making a station stop (which amtrak shouldnt). If that's not the case, then Amtrak can make the needed improvements to get it to be the case. Either way, unless you plan to build a viaduct over the NS chicago ROW or buy them out (impossible), then they need to get off the freight track as much as possible as soon as possible.